Polymer polyols, also known as filled polyols, are viscous fluids comprising fine particles dispersed in polyols. Examples of solids used include styrene-acrylonitrile co-polymers and polyureas. The solids are typically prepared by in situ polymerization of ethylenically unsaturated monomers in the base polyol. Polymer polyols are commonly used for the production of polyurethane foams, and particularly flexible polyurethane foams.
Macromers are known and have been used to stabilize polymer polyols by co-polymerization with one or more ethylenically unsaturated monomers (such as, for example, styrene and acrylonitrile). Because of similarities in chemical composition, the polyether tail(s) energetically favor association with the polyol molecules in the continuous phase rather than with the styrene-acrylonitrile co-polymer. The polyether tails extend into the continuous phase, thereby forming a “brush” layer near the particle-fluid interface which screens the attractive van der Waals forces between particles. This phenomenon is known as steric stabilization. In order to form a brush layer which effectively screens van der Waals forces several conditions must be met. The polyether tails must be similar in chemical composition to the continuous phase so that they fully extend into the continuous phase and do not adsorb to the particles. Also, the surface coverage and molecular weight must be high enough so that the interfacial brush layer is sufficiently thick to prevent agglomeration of the solid particles.
It is known that large, bulky molecules are effective macromers because less material can be used to sterically stabilize the particles. Generally speaking, this is due to the fact that a highly branched polymer has a considerably larger excluded volume than a linear molecule (such as, e.g., a monol), and therefore less of the branched polymer is required. Coupling multi-functional polyols with polyisocyanates is also known and described in the field of polymer polyols as a suitable means to increase the molecular weight of the macromer.
Preformed stabilizers (PFS) are known to be useful for preparing polymer polyols having a lower viscosity at a high solids content. In general, a preformed stabilizer is an intermediate obtained by reacting a macromer which contains reactive unsaturation (e.g. acrylate, methacrylate, maleate, etc.) with a monomer (i.e. acrylonitrile, styrene, methyl methacrylate, etc.), optionally in a diluent or a solvent (i.e. methanol, isopropanol, toluene, ethylbenzene, polyether polyols, etc.) to give a co-polymer (dispersion having e.g. a low solids content (e.g. <20%), or soluble grafts, etc.). Thus, in the preformed stabilizer process, a macromer is reacted with monomers to form a co-polymer composed of macromer and monomers. These co-polymers comprising a macromer and monomers are commonly referred to as preformed stabilizers (PFS). Reaction conditions may be controlled such that a portion of the co-polymer precipitates from solution to form a solid. In many applications, a dispersion having a low solids content (e.g., 3 to 15% by weight) is obtained. Preferably, the reaction conditions are controlled such that the particle size is small, thereby enabling the particles to function as “seeds” in the polymer polyol reaction.
It has surprisingly been found that, by incorporating carbon disulfide into the molecule during the polyether polyol synthesis, the resulting dithiocarbonate (or xanthate) group can reduce or eliminate the amount of reactive unsaturation which is necessary in the macromer in order for the macromer to be effective in stabilizing polymer polyols and/or in forming preformed stabilizers for polymer polyols.